Water Could Hold Answer to Graphene Nanoelectronics

Researchers at Rensselaer Polytechnic Institute
Use Water to Open, Tune Graphene’s Band Gap

Researchers at Rensselaer Polytechnic
Institute developed a new method for using water to tune
the band gap of the nanomaterial graphene, opening the
door to new graphene-based transistors and
nanoelectronics. In this optical micrograph image, a
graphene film on a silicon dioxide substrate is being
electrically tested using a four-point probe.

Researchers at Rensselaer Polytechnic Institute developed a
new method for using water to tune the band gap of the
nanomaterial graphene, opening the door to new graphene-based
transistors and nanoelectronics.

By exposing a graphene film to humidity, Rensselaer
Professor Nikhil Koratkar and his
research team were able to create a band gap in graphene — a
critical prerequisite to creating graphene transistors. At the
heart of modern electronics, transistors are devices that can
be switched “on” or “off” to alter an electrical signal.
Computer microprocessors are comprised of millions of
transistors made from the semiconducting material silicon, for
which the industry is actively seeking a successor.

Graphene, an atom-thick sheet of carbon atoms arranged like
a nanoscale chain-link fence, has no band gap. Koratkar’s team
demonstrated how to open a band gap in graphene based on the
amount of water they adsorbed to one side of the material,
precisely tuning the band gap to any value from 0 to 0.2
electron volts. This effect was fully reversible and the band
gap reduced back to zero under vacuum. The technique does not
involve any complicated engineering or modification of the
graphene, but requires an enclosure where humidity can be
precisely controlled.

“Graphene is prized for its unique and attractive mechanical
properties. But if you were to build a transistor using
graphene, it simply wouldn’t work as graphene acts like a
semi-metal and has zero band gap,” said Koratkar, a professor
in the Department of Mechanical,
Aerospace, and Nuclear Engineering at Rensselaer. “In this
study, we demonstrated a relatively easy method for giving
graphene a band gap. This could open the door to using graphene
for a new generation of transistors, diodes, nanoelectronics,
nanophotonics, and other applications.”

Results of the study were detailed in the paper “Tunable
Band gap in Graphene by the Controlled Adsorbtion of Water
Molecules,” published this week by the journal Small.
See the full paper at: http://dx.doi.org/10.1002/smll.201001384

In its natural state, graphene has a peculiar structure but
no band gap. It behaves as a metal and is known as a good
conductor. This is compared to rubber or most plastics, which
are insulators and do not conduct electricity. Insulators have
a large band gap — an energy gap between the valence and
conduction bands — which prevents electrons from conducting
freely in the material.

Between the two are semiconductors, which can function as
both a conductor and an insulator. Semiconductors have a narrow
band gap, and application of an electric field can provoke
electrons to jump across the gap. The ability to quickly switch
between the two states — “on” and “off” — is why semiconductors
are so valuable in microelectronics.

“At the heart of any semiconductor device is a material with
a band gap,” Koratkar said. “If you look at the chips and
microprocessors in today’s cell phones, mobile devices, and
computers, each contains a multitude of transistors made from
semiconductors with band gaps. Graphene is a zero band gap
material, which limits its utility. So it is critical to
develop methods to induce a band gap in graphene to make it a
relevant semiconducting material.”

The symmetry of graphene’s lattice structure has been
identified as a reason for the material’s lack of band gap.
Koratkar explored the idea of breaking this symmetry by binding
molecules to only one side of the graphene. To do this, he
fabricated graphene on a surface of silicon and silicon
dioxide, and then exposed the graphene to an environmental
chamber with controlled humidity. In the chamber, water
molecules adsorbed to the exposed side of the graphene, but not
on the side facing the silicon dioxide. With the symmetry
broken, the band gap of graphene did, indeed, open up, Koratkar
said. Also contributing to the effect is the moisture
interacting with defects in the silicon dioxide substrate.

“Others have shown how to create a band gap in graphene by
adsorbing different gasses to its surface, but this is the
first time it has been done with water,” he said. “The
advantage of water adsorption, compared to gasses, is that it
is inexpensive, nontoxic, and much easier to control in a chip
application. For example, with advances in micro-packaging
technologies it is relatively straightforward to construct a
small enclosure around certain parts or the entirety of a
computer chip in which it would be quite easy to control the
level of humidity.”

Based on the humidity level in the enclosure, chip makers
could reversibly tune the band gap of graphene to any value
from 0 to 0.2 electron volts, Korarkar said.

Along with Koratkar, authors on the paper are
Theodorian Borca-Tasciuc, associate professor in the
Department of Mechanical, Aerospace, and Nuclear Engineering at
Rensselaer; Rensselaer mechanical engineering graduate student
Fazel Yavari, who was first author on the paper; Rensselaer Focus Center New
York Postdoctoral Research Associate Churamani Gaire; and
undergraduate student Christo Kritzinger. Co-authors from Rice
University are Professor Pulickel
M. Ajayan; Postdoctoral Research Fellow Li Song; and
graduate student Hemtej Gulapalli.

This study was supported by the Advanced Energy Consortium
(AEC), National Institute of Standards and Technology (NIST)
Nanoelectronics Research Initiative, and the U.S. Department of
Energy Office of Basic Energy Sciences (BES).

For more information on Koratkar’s graphene research at
Rensselaer, visit: